Date:   Thu, 28 Mar 2019 18:00:21 -0400
From:   Paul Moore <paul@...l-moore.com>
To:     Richard Guy Briggs <rgb@...hat.com>
Cc:     Ondrej Mosnacek <omosnace@...hat.com>, nhorman@...driver.com,
        linux-api@...r.kernel.org, containers@...ts.linux-foundation.org,
        LKML <linux-kernel@...r.kernel.org>,
        David Howells <dhowells@...hat.com>,
        Linux-Audit Mailing List <linux-audit@...hat.com>,
        netfilter-devel@...r.kernel.org,
        "Eric W . Biederman" <ebiederm@...ssion.com>,
        Simo Sorce <simo@...hat.com>, netdev@...r.kernel.org,
        linux-fsdevel@...r.kernel.org, Eric Paris <eparis@...isplace.org>,
        "Serge E. Hallyn" <serge@...lyn.com>
Subject: Re: [PATCH ghak90 V5 09/10] audit: add support for containerid to
 network namespaces

On Thu, Mar 28, 2019 at 5:40 PM Richard Guy Briggs <rgb@...hat.com> wrote:
> On 2019-03-28 11:46, Paul Moore wrote:
> > On Wed, Mar 27, 2019 at 9:12 PM Richard Guy Briggs <rgb@...hat.com> wrote:
> > >
> > > On 2019-03-27 23:42, Ondrej Mosnacek wrote:
> > > > On Fri, Mar 15, 2019 at 7:35 PM Richard Guy Briggs <rgb@...hat.com> wrote:
> > > > > Audit events could happen in a network namespace outside of a task
> > > > > context due to packets received from the net that trigger an auditing
> > > > > rule prior to being associated with a running task.  The network
> > > > > namespace could be in use by multiple containers by association to the
> > > > > tasks in that network namespace.  We still want a way to attribute
> > > > > these events to any potential containers.  Keep a list per network
> > > > > namespace to track these audit container identifiiers.
> > > > >
> > > > > Add/increment the audit container identifier on:
> > > > > - initial setting of the audit container identifier via /proc
> > > > > - clone/fork call that inherits an audit container identifier
> > > > > - unshare call that inherits an audit container identifier
> > > > > - setns call that inherits an audit container identifier
> > > > > Delete/decrement the audit container identifier on:
> > > > > - an inherited audit container identifier dropped when child set
> > > > > - process exit
> > > > > - unshare call that drops a net namespace
> > > > > - setns call that drops a net namespace
> > > > >
> > > > > See: https://github.com/linux-audit/audit-kernel/issues/92
> > > > > See: https://github.com/linux-audit/audit-testsuite/issues/64
> > > > > See: https://github.com/linux-audit/audit-kernel/wiki/RFE-Audit-Container-ID
> > > > > Signed-off-by: Richard Guy Briggs <rgb@...hat.com>
> > > > > ---
> > > > >  include/linux/audit.h | 19 ++++++++++++
> > > > >  kernel/audit.c        | 86 +++++++++++++++++++++++++++++++++++++++++++++++++--
> > > > >  kernel/nsproxy.c      |  4 +++
> > > > >  3 files changed, 106 insertions(+), 3 deletions(-)
> > > > >
> > > > > diff --git a/include/linux/audit.h b/include/linux/audit.h
> > > > > index fa19fa408931..70255c2dfb9f 100644
> > > > > --- a/include/linux/audit.h
> > > > > +++ b/include/linux/audit.h
> > > > > @@ -27,6 +27,7 @@
> > > > >  #include <linux/ptrace.h>
> > > > >  #include <linux/namei.h>  /* LOOKUP_* */
> > > > >  #include <uapi/linux/audit.h>
> > > > > +#include <linux/refcount.h>
> > > > >
> > > > >  #define AUDIT_INO_UNSET ((unsigned long)-1)
> > > > >  #define AUDIT_DEV_UNSET ((dev_t)-1)
> > > > > @@ -99,6 +100,13 @@ struct audit_task_info {
> > > > >
> > > > >  extern struct audit_task_info init_struct_audit;
> > > > >
> > > > > +struct audit_contid {
> > > > > +       struct list_head        list;
> > > > > +       u64                     id;
> > > > > +       refcount_t              refcount;
> > > >
> > > > Hm, since we only ever touch the refcount under a spinlock, I wonder
> > > > if we could just make it a regular unsigned int (we don't need the
> > > > atomicity guarantees). OTOH, refcount_t comes with some extra overflow
> > > > checking, so it's probably better to leave it as is...
> > >
> > > Since the update is done using rcu-safe methods, do we even need the
> > > spin_lock?  Neil?  Paul?
> >
> > As discussed, the refcount field is protected against simultaneous
> > writes by the spinlock that protects additions/removals from the list
> > as a whole so I don't believe the refcount_t atomicity is critical in
> > this regard.
> >
> > Where it gets tricky, and I can't say I'm 100% confident on my answer
> > here, is if refcount was a regular int and we wanted to access it
> > outside of a spinlock (to be clear, it doesn't look like this patch
> > currently does this).  With RCU, if refcount was a regular int
> > (unsigned or otherwise), I believe it would be possible for different
> > threads of execution to potentially see different values of refcount
> > (assuming one thread was adding/removing from the list).  Using a
> > refcount_t would protect against this, alternatively, taking the
> > spinlock should also protect against this.
>
> Ok, from the above it isn't clear to me if you are happy with the
> current code or would prefer any changes, or from below that you still
> need to work it through to make a pronouncement.  It sounds to me you
> would be ok with *either* spinlock *or* refcount_t, but don't see the
> need for both.

To be fair you didn't ask if I was "happy" with the approach above,
you asked if we needed the spinlock/refcount_t.  I believe I answered
that question as comprehensively as I could, but perhaps you wanted a
hard yes or no?  In that case, since refcount_t is obviously safer, I
would stick with that for now just to limit the number of possible
failures.  If someone smarter than you or I comes along and
definitively says you are 100% safe to use an int, then go ahead and
use an int.

Beyond that, I'm still in the process of reviewing your patches, but I
haven't finished yet, so no "pronouncement" or whatever you want to
call it.

-- 
paul moore
www.paul-moore.com

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